As a device for detecting step-out of an electric power system, for example, a device described in Japanese Patent Laying-Open No. 2007-60870 (Patent Document 1) has been known. In a plurality of electric power systems each including at least one generator and bus and coordinated with each other by connecting the buses via a link line, the device predicts step-out of the generators. In particular, the device predicts that step-out will occur if the generators continue operation, based on a voltage of a bus and a current flowing from the link line to the bus.    Patent Document 1: Japanese Patent Laying-Open No. 2007-60870